In order to produce machine parts capable of serving their individual, characteristic functions, a casting method has been conventionally and extensively employed in joining the first and second parts which are formed of metallic materials having different properties (in this specification, the first part is a part to which another part is to be joined and the second part is a part to be joined to the first part). However, such a casting method is limited in its application when the second part is made from an alloy. Because it is inevitably required in the casting method to completely melt the second part, and in the case that the second part is made from an alloy and this alloy especially contains a metal having a low-boiling point, such a metal vaporizes during the melting of the entire alloy. This causes such a phenomenon that when the second part is made from an alloy of copper and zinc for example, zinc vaporizes just before the melting point of the alloy and forms a layer of zinc or zinc oxide on the surface of the second part. This phenomenon reduces the bonding strength of the second part at the joint face of the first part, or causes a difference between the copper-zinc composition ratio of the second part and that of the casting material. Therefore, it has been thought to be very difficult to unite the second part made from an alloy to the surface of the first part by means of the casting method.
Diffusion welding is a method of uniting parts by not melting but applying pressure to the second part so as to be joined to the first part. Like the ultra-high pressure welding method and the HIP method, in which ultra-high pressure (hydrostatic pressure in the case of the HIP method) is applied to the first and second parts which have been superposed, in the coaxial superposed direction thereof, diffusion welding is suitably employed for the second part made from one of the above-mentioned materials that are difficult to unite by casting.
Of the above welding techniques taken in a solid phase condition, the hot diffusion welding method, in which an insert material is interposed between the first and second parts and each part is subjected to heating and pressurizing so that they are united, can be the most promising in achieving excellent bonding effects by suitably selecting an insert material.
In the hot diffusion welding method, it is common to utilize resistance heat as heating means, the resistance heat being generated by supplying a current. One example of such a technique conventionally used can be seen in the welding of a sliding member for a hydraulic apparatus. According to this conventional technique, as shown in FIG. 3, a brazing filler metal of normal type 33 serving as the insert material is interposed between a first part 31 such as a shoe formed of a SCM material and a second part 32 such as a sliding member formed of a PBC material, thereby forming a body in which the first part 31, the brazing filler metal 33 and the second part 32 are superposed. Then, the hot diffusion welding method is performed in such a way that while a pressure 38 being applied to the superposed body thus obtained, the superposed body is heated by supplying a current from a power source 39 through a copper electrode 34 and another copper electrode 35. In the above-described conventional hot diffusion welding method, heat is not easily transferred between the first part 31 and the second part 32 because of the interposition of the normal brazing filler metal 33. Further, since the second part 32 is generally thin in its shape and has a small thermal capacity, Joule heat generated at the second part 32 is transmitted to the copper electrode 34 when the second part 32 is brought into direct contact with the copper electrode 34 which has been water-cooled, with the result that the joint faces cannot be heated to a temperature which is high enough to carry out diffusion welding with the melting behaviour of the brazing filler metal 33 interposed between the joint faces. It is, therefore, unavoidably necessary to provide an auxiliary electrode 36 of doughnut shape serving as a heat insulating material between the copper electrode 34 and the second part 32, as well as an electrode such as a silver-tungsten electrode 37 for compensating for the electric conductivity of the auxiliary electrode 36.
In the resistance heating means in the conventional technique as mentioned above, even the silver-tungsten electrode 37 is oxidized at the surface thereof as the number of welding operations increases, so that the deformation rate of the silver-tungsuten electrode 37 which is in contact with the surface of the second part 32 increases as well with the increased welding operations. This causes such a trouble that it becomes necessary to polish the silver-tungsten electrode 37 to remove the oxide film formed thereon when the deformation rate exceeds a certain level. Furthermore, electrodes other than the copper electrodes 34 and 35, namely, the auxiliary electrode 36 and the silver-tungsten electrode 37 need to be prepared for every size of the second part 32, and this increases the complexity of the apparatus for carrying out hot diffusion welding.
In the conventional hot diffusion welding method where resistance heating is utilized as heating means, there are limitations in using a brazing filler metal as the insert material. Taking amorphous metals used as the brazing filler metal in hot diffusion welding method for example, those metals available in the market are limited in their quality and characteristics and therefore often fail in achieving a satisfactory bonding strength. Concretely, amorphous metals have the following disadvantages:
(a) most of them are in the form of metal foils having low melting points and high ductility, so that the bonding strength decreases when heat generated by friction or shear force is added;
(b) although some amorphous metals containing components such as tungsten and molybdenum have high melting points, they still have disadvantages in wettability for the adaptability with the first part and/or second part; and
(c) in the first part and/or second part, they create fragile intermetallic compounds between metals contained as impurities and metals constituting the parts.
Therefore, brazing filler metals unavoidably exhibit a poor bonding strength when used in the welding of machine parts such as sliding members of a hydraulic apparatus where high heat resistance is required and where a great shear force is applied.